How to Troubleshoot Electrical Systems

Sept. 28, 2010
How Can a Digital Voltmeter Mislead?
The digital voltmeter is the best basic tool for electrical troubleshooting, because it's calibrated and exact. But remember, the digital voltmeter can't detect a high-resistance fault if the circuit is open. You'll read full-system voltage in a high-resistance circuit unless the circuit is tested under load.
Testing for Battery Draws
Any fused circuit that has a current flow will have a small millivolt (one thousandth of a volt) drop. You can use the voltmeter set on "mV" across the fuses to test for current drain. The voltage drop will be small (5 to 20 mV) for a small current, and greater (above 20mV) if the current is large. (The numbers used here are approximate.) Remember, voltage drops occur only in wires with current flowing through them, and if current is flowing, the wire can't be open.

In previous articles for Construction Equipment magazine, I've detailed specific skills and practices that can be used to speed up and streamline electrical troubleshooting. In this article I'll be doing the same, but I'll also try to suggest a few ways you can alter your thinking about the way you approach troubleshooting.

First of all, let's assume that we want the same thing — namely, a faster process for finding electrical faults that's accurate and definitive. We want fewer wrong turns, fewer parts replaced unnecessarily, and a testing and repair process that reduces the chance we'll cause damage that will come back to haunt us in the future.

Attaining these goals is not as difficult (or impossible) as it might seem. The issues involved here are the knowledge, thought processes and confidence of the technician. First and fundamental, the technician must have a detailed understanding about how the voltmeter works and a disciplined approach to reading electrical schematics. My experience has taught me that these are absolutely critical skills. We've touched on these subjects before, but we'll cover a few more tricks in these areas in our present discussion.

But just as important is the technician's thought process. Electrical troubleshooting is a logical exercise that requires logical thinking. Too many technicians start out with hasty, incorrect assumptions; they're doomed to fail before they start. But take heart; it's a situation that can be reversed, and you'll find your confidence growing as you learn to thoughtfully exercise basic skills.

Slow down to save time

Okay, I know what you're about to say: "If I take time to think through an electrical problem, they'll say I'm not working."

But actually, you're saving time. As an electrical troubleshooter, you must keep track of time and not let it get the better of you. Without question, most technicians have a misplaced sense of time when working on electrical problems. I've seen them waste two or three hours walking around, changing parts (without knowing for certain if the parts are faulty), pulling wire harnesses apart just to look at them, and doing other "little" things that add up to hours of effort.

My suggestion for technicians who approach problems in this manner is that they sit down and read the schematic — perhaps for as little as half an hour — to orient themselves and figure out how the system works. Unfortunately, that suggestion, as we've already noted, typically is met with protest: "I can't waste the time to do that," or "I'll get yelled at if I do that."

Schematic reading is a skill that is central to the troubleshooting process, and if you try to avoid it, you'll be working on the machine as if blind. It's crucial that technicians get into the books and get the system figured out before starting the troubleshooting process. Doesn't it make sense to gain some idea of what could be causing the problem before you attempt to make the repair?

I tell students repeatedly: "You troubleshoot circuits, not wires. The circuit is simple; the wiring is complex."

My approach to troubleshooting is first to make a simple drawing of the suspect circuit in order to figure out what it does (and what it doesn't do) and, then, to repair the wires. Technicians can waste a lot of time, because they think they need to see the wiring in order to figure out what's going on. So, they rip open harnesses, cut tie-wraps and unravel wire looms. Nothing could be more unproductive. The redrawing process forces technicians to slow down and to analyze, resulting in intelligent decisions.

Dare I say it? The ohmmeter is really a terrible troubleshooting tool. Now, don't misinterpret this to mean that using the digital meter is a mistake. What I'm saying is that the OHM function on the meter is not anywhere as effective and useful as most people assume.

The voltmeter is far more effective for finding faults. It's much faster; it's more meaningful; it's more accurate; and it's safer. When the voltmeter is used properly, every reading is a specific answer. Using the voltmeter doesn't require you to know what's wrong before you start. It can test the entire circuit — connected — in just a few steps, rather than many — disconnected — steps.

But that said, the voltmeter is often misused. For instance, some technicians round off readings, instead of using the exact number: 13.3V is not 12V and 11.8V is not 12V. If you don't take advantage of the meter's accuracy, you'll miss the diagnosis. And for all of its virtues, the voltmeter can mislead you in certain situations; but if you're aware of this, you won't be tripped up. But unfortunately, some technicians haven't yet learned to use the voltmeter to its full potential, often ignoring valuable readings that they don't understand.

Specific faults/specific readings

For instance, a little-known voltmeter reading is called "ghost voltage." Digital voltmeters are so sensitive that even the earth's magnetic field will induce a very small voltage in its leads (as when you turn it on). You get the same result — a rapidly fluctuating, low voltage reading (ghost voltage) — when the leads are touching two wires that are not connected, as when there's a broken or disconnected wire stopping current flow.

This is an "open circuit." So when you see this reading, don't say, "what the heck is this?" but begin to investigate for an "open." Current flow through an open circuit is zero. The circuit is dead.

When making this investigation and you're looking for voltage at a component you have unplugged, the voltmeter will register ghost voltage if the wire is not complete (open) to the component, or exact system voltage if the wire is complete to where you are. If you do read exact voltage, you then know that something after the component is open.

On the other hand, if the meter reads 0.00V and it locks in on that reading, the correct answer is "continuity to ground," which often is expressed as a "short-to-ground." This is a specific fault (copper from the circuit touching a steel ground), and it causes the voltmeter to give a specific reading — 0.00V or "true zero voltage." Some technicians, however, don't understand this reading. If the wire you're testing is shorted to ground, then both leads will be at the same potential, and the meter reads, "zero".

A short-to-ground causes very high — possibly damaging — current, and if the fuse blows, it's to protect the wire. Whatever you do, don't install a 50-amp fuse in the panel to "find the short." If you do, you risk a fire that can burn the machine down to the tires. I've seen it happen. True zero voltage is blatant, because ghost voltage stops and zero locks in. This is how the VOLTmeter shows you continuity, making the OHMmeter unnecessary.

Tricky high resistance

The most dangerous error a technician can make is assuming that a wire is actually clean and can carry a load — just because you see a correct voltage. There's a big difference between voltage (pressure) and amperage (flow). Seeing voltage on the meter confirms that wires aren't open, but not that they will carry a load. This situation can occur if the circuit has a high-resistance fault, such as when corrosion or rust blocks current flow enough to cause the load (component) to malfunction.

Believe me, a great many parts are replaced needlessly because of this error. Since the technician reads full system voltage when testing the circuit to the component, the usual assumption is that the circuit is capable of carrying proper current; therefore, the component itself is bad. This problem is very frustrating, because you change the part with confidence after following the manufacturer's instructions, but it still doesn't work.

Unfortunately, a conventional digital voltmeter can't see this problem. The corrosion will keep the load from functioning properly, but it won't keep the voltmeter from reading 12V or 24V — that is, "normal." Light-bulb testers also will fail in this situation, especially those with LEDs, because it takes very little current for the LED to light. And in some conditions, you can't tell if the light is fully bright. Most of these light-bulb testers will actually be fully bright with as much as 10,000 ohms, and it takes only 20 ohms or less to kill a circuit.

Technicians who understand the problem of high-resistance faults and the voltmeter's inability to find the fault in a static test (that is, without current flowing) sometimes work around it by devising a dynamic test (with current flowing) by using a headlight that can be placed in the circuit to "load" it. Or, another possibility is using a testing tool capable of performing a dynamic test, such as TESlite Voltmeter Leads, which allow you to determine if the circuit is free of resistance by loading the circuit during the voltage test.

In conclusion, the best way to think about all of this is to understand that troubleshooting is a brain game, not a wrench-turning, meter-reading, parts-changing game. Once you've read the schematic, redrawn the circuit, determined what the circuit is supposed to do and, finally, have taken meter readings to see what the circuit is (or isn't) actually doing, then you troubleshoot.

Don't do something with your hands just to look busy. You'll actually make more intelligent repairs if first you sit on your hands and engage your mind.

Construction Equipment thanks Dan Sullivan for sharing his expertise in this special report. Graphics are adapted from Dan's practical handbook, Fundamental Electrical Troubleshooting.

Circuit Fault Definitions
  1. OPEN CIRCUIT: An open circuit has zero amps of flow and no voltage drop at all.
  2. SHORT-TO-GROUND: A short-to-ground creates a low-resistance circuit that bypasses the load (working component) and causes high current.
Circuit Fault Definitions
  1. OPEN CIRCUIT: An open circuit has zero amps of flow and no voltage drop at all.
  2. SHORT-TO-GROUND: A short-to-ground creates a low-resistance circuit that bypasses the load (working component) and causes high current.
  3. HIGH-RESISTANCE: High-resistance faults — which can include corroded wires, corroded connectors and bad grounds — cause low current and a strange voltage reading.

* This voltage reading assumes the circuit is being tested under load.